In optical detector systems, such as those used in liquid chromatography, sample constituents of interest are identified and quantified by characteristic spectral absorbance or fluorescence. A typical detector device will have a flowcell. The flowcell has a housing defining a chamber for receiving sample. The chamber has an entrance end and a exit end. The entrance end has a entrance opening having a rim. Similarly, the exit end has an exit opening having a rim. The rims separate the exterior of the housing from the one or more walls of the interior surface of the chamber. The openings may have optical windows or lenses. The chamber defines a vessel for receiving a sample.
Sample, held in the chamber, is subjected to light entering the chamber about the entrance opening. Light is discharged at the exit end. The spectrum is analyzed to determine absorption indicative of a chemical. To effect this, a monochromator or spectrograph must co-operate with the sample chamber of the flowcell.
The size and shape of an absorption chamber for a high performance liquid chromatography (HPLC) ultraviolet (UV) to visible (Vis) light detector is a compromise. High light flux passing through the cell is important to achieve a high signal-to-noise-ratio measurement. The cell volume must be kept low to prevent peak spreading and loss of chromatographic resolution. For a given cell volume and optical throughput, the cell path length should be as long as possible to maximize sample absorption. The dimensions of a typical chamber of a conventional HPLC-UV-Vis absorption flowcell are 10 mm long, and 1 mm in diameter. The total volume is about 8 micro liters.
The shape of the chamber is dictated by manufacturing limitations. The typical flowcell chamber is cylindrical or somewhat conical, with a circular cross-section. These chambers can be machined with straight or tapered reamers. The best optical throughput is obtained when one end of the chamber or flowcell is conjugate with the light source and the other end is, or is conjugate with, the primary aperture stop of the optical system.
A further consideration is dictated by the optical system in which the flowcell is used. When a flowcell is incorporated into a spectrometer system, the light beam which passes through the flowcell must also pass through a grating monochromator or a spectrograph. Good spectral resolution in a compact format, requires the use of a narrow slit. The best optical throughput through the monochromator or spectrograph is achieved if the two primary stops of the optical system are imaged, or conjugate with, the grating and slit. Thus the slit ends up being conjugate with (or coincident with) one end of the flowcell and the shape of the other end of the cell corresponds to the beam shape at the grating.
This has led to a problem matching the optics of the most efficient and compact spectrometer to the most efficient flowcell. Typically, one end of the flowcell (round) is imaged onto the spectrometer slit (tall and narrow), and light is lost on the sides of the slit. Alternatively, one end of the round flowcell is the slit. The result is either poor spectral resolution, or low light throughput.
It is desirable to avoid complicated arrays of mirrors and fiber optics to address the problems of matching the optics of the flowcell to the monochromator or spectrograph.